std error of multiple dummy regression


I've run some multiple regression on R. The indepensent variables are all dummies plus an intercept.
The results are good, and I've noted that the std error of all the dummies coefficients are equal.
Could somebody explain me the reason?
I expect something linked with the fact that all the dependent variables have the same variability...



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What are the dummy variables representing? Can an observation only be a part of one of the dummy variables? Are there an equal number of observations for each dummy variable?